Hi @mariestorko
The issue in your gallery seems to be the thumbnails were not created properly or they got corrupted.
Go to NextGEN Gallery > Other Options > Miscellaneous and click “Clear Image Cache”. Then go and load the gallery again, wait a bit for the thumbnails to be recreated.
Please go to Tools > Site Health > Into Tab (at the top), then click Copy Site Info to Clipboard. Paste the info in a notepad (CTRL+V or CMD +V) and check for execution times and time limits, for example:
time_limit: 120
max_input_time: 60
both recommended to be set at 300
Also some other limits that might affect image processing like these:
memory_limit: 128M
recommended at least 250
upload_max_filesize: 512M
php_post_max_size: 512M
these should be around what you normally upload to your site
You can ask your hosting company to adjust accordingly
Please keep me posted.